A bi-directional valve which is operated by applying an external force to a flexible element, or diaphragm (typically an elastomer). Diaphragm valves may be used for slurries (where other valve designs might clog) or in hygienic applications.
A valve which can change the direction of the flow of a medium to two or more different directions.
Double block and bleed
A valve configuration in which positive shut-off is achieved at both the inlet and outlet sides. A small port is fitted to discharge fluid in the intermediate space. Fitting a gas detector to the port provides assurance of the integrity of the upstream seal. This configuration is often required to isolate high pressure sections of a system to facilitate safe maintenance, etc.